The nail plate (i.e., the natural nail) is primarily composed of keratin, a water-insoluble, fibrous protein that is a major structural component of skin, hair, wool, silk, feathers, scales, nails and hooves. While keratins can obviously differ greatly in their amino acid makeup, hard keratins may all be generally characterized as cross-linked polypeptides. Alpha-keratins such as nails and hooves may be further characterized by their relatively higher percentages of the amino acid cysteine. Typically, the alpha-helix coils of the polypeptides are cross-linked with disulphide bonds between adjacent cysteines. The resulting plate-like cells are cemented to each other with a sticky substance and held together by rivet-like structures called desmosomes. Many cell layers adhere to each other to form the nail plate—a structure that resembles a brick and mortar wall.
Primers are adhesion promoters that improve adhesion by increasing interfacial compatibility between surfaces, e.g., the nail plate and an applied coating. For example, a coating of nail polish will resist chipping and peeling if a good base coat is used. Base coats are more compatible with the nail plate than the nail polish. Base coats act as the “go-between” or “anchor”, to improve adhesion.
Primers are also frequently used with artificial nail enhancements since acrylic nail products normally have poor adhesion to nail plates. In general, nail plate primers can be thought of as double-sided sticky tape, joining the nail plate to the nail enhancement. The nail plate surface is made up of chemical groups possessing specific structures. Primer molecules must match the chemical and structural characteristics of the nail plate. More particularly, one end of the primer is reactive with the methacrylate monomers. With these types of primers, physical abrasion of the nail plate is required to achieve proper levels of adhesion to the keratin substrate. Moreover, these acids are corrosive, and if used improperly they can cause damage to the nail plate and surrounding tissue. These acids can also cause discoloration of the nail enhancement and are a leading cause of nail product discoloration. This invention eliminates a large percentage of discoloration problems for professional nail technicians. But even more importantly, in response to a number of chemical burn injuries, primarily to children, the Consumer Product Safety Commission recently issued a regulation requiring child-resistant packaging for all household products containing more than 5% methacrylic acid. However, child-resistant caps increase the risk of spills in the salon as Nail Professionals struggle to remove the cap. This invention solves both the burn injury and child-resistant cap issues because it utilizes a non-corrosive solvent, while still providing the desired adhesion properties.
Commercially available nail primers rely solely on hydrogen bonding. Hydrogen bonding on organic substrates such as keratin typically depends on the interaction between an oxygen or nitrogen atom that is covalently bonded to the upper surface of the nail plate and a hydrogen atom, covalently bonded to methacrylic acid, which is covalently linked to the polymer. A special type of interaction called a hydrogen bond exists between the interfaces of these dissimilar surfaces. Hydrogen bonds are types of attractive, intermolecular bonds that are characteristic of atoms with high electonegativity, i.e. fluorine, oxygen, sulfur, and nitrogen. They are many times weaker than the weakest covalent bond, which is found between a carbon and acidic hydrogens such as C—H as found in chloroform and acetylene. This weakness accounts for the attraction between the acidic hydrogen and a nearby organic, acidic hydroxyl group of acrylic or methacrylic acid primer, as well as the inherent relative weakness of hydrogen bonds. The overall strength of the hydrogen bond is determined by the strength of this relatively weak carbon/hydrogen bond. It is a controlling factor in hydrogen bond strength. Therefore, when acidic primers are used, the weakest adhesive link will exist between an oxygen molecule on the keratin surface and the acidic hydrogen of (meth)acrylic acid. Since covalent bonds are many times stronger than hydrogen bonding, improvements in adhesive bond strength can be achieved by eliminating the hydrogen bond and replacing it with a stronger, more permanent, organic covalent bond.
It is clear from the foregoing that there are three fundamental problems with currently available methacrylic primers and acrylic acid adhesion promoters. First is the corrosive nature of their primary component, methacrylic acid. Second, they create temporary hydrogen bonds that are inherently weaker than covalent bonds, leading to a weaker interfacial adhesive bond between the natural nail plate and the primer molecule, with a stronger adhesive bond between the primer molecule and the polymer chain of the nail enhancement. Third, acid-based primers are a primary cause of nail enhancement discoloration. Fourth, acid-based primers can result in chemical burn injuries.